A microscopic DEM investigation on fracture shearing characteristics of infilled grains with different geometrical shapes in rock discontinuities

Abstract

Rock discontinuities or faults often contain a layer of granular material. However, the evolutionary behavior (movement and breakage) of such infilled grains under shearing have not been comprehensively studied. To better understand this issue microscopically, numerical direct shear tests are performed on small rock discontinuity with single-grain infilled under different normal stresses by PFC^2D, with emphasis on the effects of grain geometry (reflected by the aspect ratio (a/b)) and shear rate. Under the low normal stress (i.e., 0.1 ​MPa), circular grains (a/b ​= ​1.0) undergo in pure rolling during the shear process, with slight surface erosion, and the shear stress remains almost constant except for several fluctuations. The movement of grains with larger a/b changes from rolling to sliding or even crushing as the shear displacement increases. Under the high normal stress (i.e., 0.6 ​MPa), grains can eventually be crushed into a few large angular fragments and many fine comminuted particles, accompanied by severe damage to discontinuity surfaces, significant shear shrinkage, and violently fluctuating shear stress. The volume fraction of large angular fragments increases with the increase in a/b value, while that of fine comminuted particles decreases. Shear rate also has a significant impact on grain behavior. The main movement of grain with a/b ​= ​2.0 changes from rolling to sliding and even crushing under the low normal stress with the increase in shear rate. Rock discontinuities exhibit unstable shearing, and surface damage is less significant under the high normal stress and higher shear rate. The dominant failure mode in grains and discontinuity surfaces involves tension microcracks at different shear rates, while tension microcracks in the grain under high normal stress decrease drastically as the shear rate increases. Effects of micro-parameters of infilled grain are also investigated through sensitivity analysis. The observations provide implications for the macro-shear mechanism of rock discontinuity infilled with granular materials.